Colligative+Properties

Colligative properties are properties of solutions that are related to the number of particles of solute rather than to the chemical identity of these solute particles. These solution properties include vapor pressure, boiling point, and freezing point.

Vapor pressure is caused by the evaporation of molecules at the surface of a liquid. These escaping molecules exert an upward pressure as they leave the liquid. For example, gasoline has a greater vapor pressure than syrup because molecules of gasoline evaporate more readily than molecules of syrup. Perfume and alcohol have vapor pressures that are greater than the vapor pressure of water. The vapor pressure of a liquid is reduced by the presence of solte particles in the liquid because the solute molecules reduce the number of the liquid molecules on the surface. It is the surface molecules that evaporate and therefore create vapor pressure. The vapor pressure of water is greater than the vapor pressure of a sugar solution because the presence of sugar molecules at the liquid's surface decreases the number of water molecules that are free to evaporate.
 * Vapor Pressure**

The boiling point is the temperature at which the vapor pressure escaping from a liquid is equal to the atmospheric pressure pushing down on the surface of the liquid. //Boiling point elevation// means that the boiling point of a liquid increases when molecules of a solute are added. The reason for this elevation in boiling point is that the presence of the solute decreases the vapor pressure of the liquid; therefore the liquid has to be heated to a higher temperature in order to have enough vapor pressure to match the pressure of the atmosphere.
 * Boiling Point**

When a liquid freezes, its molecules slow to the point that they no longer slide past each other. The molecules of a frozen liquid are arranged in a structure that does not have the freedom of the liquid state. If particles of solute are added to a liquid, the solute particles interfere with the orderly alignment of solvent molecules as freezing takes place. The temperature of the liquid must then be lowered even more before freezing can take place. This is called //freezing point depression//, which means that a liquid with a solute in it freezes at a temperature lower than that at which the liquid alone would freeze.
 * Freezing Point**

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